At present, an image is transmitted from a computer to a display in such a manner as to transmit a raster image with respect to each frame frequency. This requires a large amount of transmission data, and involves unnecessary transmission when a still image is being displayed.
As one approach to reducing the amount of transmission data, an image may be compressed into a file format such as JPEG or GIF, and then transmitted. However, this approach requires a processor which performs high-speed operation for compressing and decompressing the image with respect to each frame, resulting in an increase in cost.
There may be used a different approach than the image compression, which involves reducing the bit-plane number of a raster image. The bit-plane number herein means the bit number “n”: the number of bits of data representing the tone or gray level of a digital image which has been quantized by 2n. Examples of the methods for reducing the bit-plane number include the multi-level dither method and the fixed threshold method. A detailed description of these methods is found in “The New Image Electron Handbook” (Tokyo, Corona Publishing Co. Ltd., 1993), pp. 41-51. The multi-level dither method and the fixed threshold method differ from the image compression method utilizing a format such as JPEG or GIF in that compressed images do not need decompression.
The conventional multi-level dither method and the fixed threshold method, however, have some problems as follows.
1. The reduction of the bit-plane number causes false contours, false colors and graininess or granularity, thus deteriorating the quality of an image.
2. In the case of superimpose display (a technique for superimposing a different image such as “text” on a displayed screen image), a plurality of images (e.g. a picture or a graphic and text) are necessary as input images, which increases the amount of input image data. Therefore, it becomes difficult to store the input images in a memory and transmit them via a transmission bus which has a limitation in bus width.
3. With a display of, for example, a mobile terminal, having a display screen of low maximum resolution, it is necessary to scroll the content of the screen when an image displayed thereon is large as a map. This scroll display is deceptively simple operation. However, a display memory has to be rewritten many times, and accordingly, electric power consumption is increased.
4. In the case of dithering for a raster image, a high-frequency minimal noise can be obtained as the dither period becomes shorter, and it is possible to reduce deterioration in picture quality. However, the number of pixels in the main scanning direction of a display is generally a number including “2” to “6” as a factor (480, 720, 840, etc.). Therefore, image quality deteriorates through compression and decompression of the image. When the dither period is set to a larger value so as not to be a factor of the number of pixels in the main scanning direction of a display, a high-frequency minimal noise, the intended purpose of dither processing, cannot be achieved. Thus, image quality deteriorates by compression and decompression of the image.
In Japanese Patent Application laid open No. 2003-162272 (Reference 1), there is disclosed a conventional technique entitled “Image processing apparatus, Image transmission apparatus, Image reception apparatus and Image processing method” for solving the problems.
FIG. 1 is a diagram showing an example of the construction of the conventional image processing apparatus. In the image processing apparatus, an input image is dithered first according to the X and Y coordinates of the pixel, and then quantized to be stored in a memory. The data read out from the memory is subjected to inverse quantization. Thereafter, the same dither matrix as used for dithering the input image is added to the data to output it to a display.
The image processing apparatus, however, causes a distortion of 0.5 in the gray level of an image before and after dithering. Consequently, the image after dithering becomes brighter by a gray level of 0.5 as compared to that before dithering.
The gray-level distortion or change is particularly distinguishable when dithered images and non-dithered images are displayed alternately.
To correct gray level for the distortion of 0.5, an offset of 0.5 may be added to the image signal.
FIG. 2 is a diagram showing another example the construction of the conventional image processing apparatus in which an offset is added to an image signal on the output side. In this case, possible dither values are 0, 1, 2 and 3, while an offset is 0 or −1 which is to be added to an image signal to correct gray-level distortion.
That is, one of the values −1, 0, 1, 2 and 3 is added to an image signal after inverse quantization, and the additional value can be either positive or negative. Therefore, the addition of an offset to an input signal requires not an adder circuit but an adder-subtractor circuit. However, compared to an adder circuit, the size of an adder-subtractor circuit is larger by at least 20%, and the circuit size inevitably increases.
FIG. 3 is a diagram showing another example the construction of the conventional image processing apparatus in which an offset is added to an image signal on the input side. In this case, a value to be added to an image signal is 0 or −1. Accordingly, a subtractor circuit can be employed without need for an adder-subtractor circuit. In other words, if an offset is added to an image signal on the input side, an increase in circuit size can be suppressed.
FIG. 4 is a diagram showing examples of offset matrices to be added to a dither matrix to correct distortion between input and output signal values. In the case of Offset Example 1 in FIG. 4, the average gray level of output signals is 16.5, and the problem of gray level distortion cannot be solved. On the other hand, in the case of Offset Example 2, the average gray level of output signals is 17, and the problem of gray level distortion can be solved. That is, by selecting an appropriate offset, no gray-level distortion occurs.
FIG. 5 is a diagram showing distortions in output values in the case of no offset. FIG. 6 is a diagram showing distortions in output values in the case where an offset matrix, indicated as Offset Example 2 in FIG. 4, is added to a conventional threshold matrix to perform bit addition. As can be seen in FIG. 5, when no offset is applied, the same output value appears for every two pixels in each row. Meanwhile, as shown in FIG. 6, when Offset Example 2 is applied, the same output value appears for every four pixels in the first and third rows.
Generally, when at a low frequency, noise components are human-perceivable. Therefore, the application of Offset Example 2 deteriorates image quality.
In other words, by simply adding an offset to an image signal on the input side, gray-level distortion cannot be corrected. Even if gray-level distortion can be corrected, cyclic noise is generated, which deteriorates image quality.
As just described, the conventional technique has some problems in that the correction of distortion in gray level before and after dithering necessitates the increase of circuit size and the generation of cyclic noise which causes image deterioration.